Recently, the use of hazardous metals such as lead, mercury, cadmium, and hexavalent chromium has been restricted by environmental regulations such as RoHS (Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment) and ELV (End of Life Vehicles) Regulations.
In accordance with this movement, a chromate film, which is effective as an anticorrosive chemical conversion film for a member having a metallic surface such as a galvanized member, has been formed not by a composition for chemical conversion treatment, which may be referred to below as a chemical conversion treatment solution, using a chromate salt containing hexavalent chromium but by a chemical conversion treatment solution containing trivalent chromium.
Some chemical conversion treatment solutions are capable of forming a chemical conversion film having a black appearance, which may be referred to as a black film. Such chemical conversion treatment solutions are mainly used for treating members and parts for office equipment, electric appliances, and vehicles. Examples of such members include plates, housings, hinges, and press molded parts such as panels. Examples of such parts include fastening parts such as bolts and nuts, and attaching parts such as clumps and clips. However, there is a problem such that chemical conversion treatment solutions are not stable. Therefore, the appearance of a black film formed from a chemical conversion treatment solution, the total area of which is increased, is deteriorated, namely, its appearance becomes gray. The total area of the black film formed from a chemical conversion treatment solution may be referred below to as the total treated area of a chemical conversion film.
In order to overcome the above-described problem, a method of adding a sulfur compound to a chemical conversion treatment solution was proposed, as described in Patent Document 1.    Patent Document 1: JP2005-206872A